I don't know but i wouldn't have thought so.
EDIT: found this. it explains the reason why not:
Hi all, we've set this idea to Not Suitable because you have to go online to use paypal anyway, and to top up paypal you need a credit card as well, so why not simply use the services we've already got in place. Aside from that, there is a cost involved for both giffgaff and the members who use paypal, and we'd rather keep the costs low.
I think it would be a good feature to have.
I understand the "you've got to go on-line anyway" argument.
Yes that's true but some people might like the extra security that paypal brings.
I've recently given a SIM card to a friend as she's currently assessing mobile networks, however one of her criteria was that she didn't want to have to put her debit/credit card information on-line. Turns out that she's had a couple of bad experiences with on-line payments where she's been a victim of card fraud.
I know that you can top up via voucher - which is fine, but giffgaff vouchers aren't easy to find (or at least, not easy to get shop assistants to go to the bother of finding something that's not on the first screen) , O2 vouchers work, but take a while to clear. So while it's totally doable that way it wont be a slick experience.
I dont have any problem myself, I trust that giffgaff has good security on card details, but giffgaff is not (yet) a household brand and some people might have understandable reservations for putting their details in, especially if, like my friend they have had a bad experience previously.
A paypal option would give such people certainty, paypal has that reputation of being secure and trustworthy.
I can see the arguments from both side.
I'm sure giffgaff have their reasons, transaction fee probably being one of them, but if there was a way to overcome those hurdles then I'm sure having that extra option would be welcomed by many.